1. Technical Field
This invention relates generally to micro-electrical-mechanical systems (MEMs), and more particularly to active MEMs incorporated into bolometers for detecting electromagnetic radiation between about 90 GHz and about 30 THz.
2. Description of the Background Art
There have been two previously unrelated issues, each in a different field. The first issue has been a need for bolometers that could image at lower frequencies at room temperature. The second issue, unrecognized until now, is a need for MEMs systems incorporating active components, or active MEMs. One aspect of the present invention is to solve the need for improved bolometers by manufacturing bolometers that incorporate active MEMs.
The relative advantages of utilizing electromagnetic radiation sensors, specifically quantum detectors and bolometers, for specialized imaging applications are widely recognized. Quantum detectors are non-equilibrium devices that respond to the quantum nature of electromagnetic radiation and produce signals proportional to the number of photons received from a scene. Typically, for proper operation, quantum detectors designed for sensing electromagnetic radiation below 300 THz should be cooled. Bolometers, on the other hand, are equilibrium devices whose signal depends on the difference between the power received from the scene and lost to the environment through radiation, convection and/or thermal conduction. Since bolometers operate in a vacuum, the power the absorber stage, also known as the detector stage, looses to the environment is lost through radiation and thermal conduction through supporting bridges from the absorber to the environment. The absorber in bolometers is thermally isolated from the environment and typically bolometers are operated at room temperature.
While progress has been made in the development of quantum detector-based LWIR (30 THz) and MWIR (70 THz) imaging systems, the need for cooling has always complicated system design, increased weight, reduced reliability, and increased cost. The cooling problems become even more pronounced when attempting to image at lower frequencies (or longer wavelengths) for example, between about 100 GHZ and 1 THz. For this reason, there is much interest in developing bolometers imaging at room temperatures, such as between 90 GHz and 30 THz.
A bolometer based imager is typically made up of a plurality of bolometer pixels assembled into an x-y array with associated readout circuits. Each pixel includes a passive absorber element mechanically supported above a heat bath by bridges that also provide passive thermal isolation. Bolometers with passive thermal isolation have been used to construct LWIR imaging systems that operate at room temperature; however, because of insufficient thermal isolation, these systems have performed at least an order of magnitude below their theoretical sensitivity limit.
The issue of thermal isolation for imagers imaging between about 90 GHz and about 1 THz (that is at longer wavelengths) is more critical, resulting in further sensitivity degradation. To overcome limitations inherent in conventional passive thermal isolation bridge designs it has been proposed to utilize active thermal isolation to minimize thermal loading on the absorber element. Active thermal isolation utilizes electro-thermal feedback to adjust the temperature of an intermediate stage to be equal to the temperature of the absorber element, e.g., as disclosed in U.S. Pat. No. 6,489,615, incorporated herein by reference. Actively equalizing the temperature of an intermediate stage with the absorber stage creates a situation where almost zero net thermal current flows between the absorber stage and intermediate stage, or providing almost ideal thermal isolation.
Mechanizing electro-thermal feedback requires incorporating special active circuits within each bolometer pixel. These special active circuits are formed in isolated single crystal silicon island interconnected by thermally insolating mechanical/electrical bridges. Specifically, electro-thermal feedback in each pixel requires: (1) temperature sensors, (2) a temperature difference amplifier, (3) a heater with an output dependent on temperature difference, and (4) a structure which incorporates these items with an absorber element into a single pixel.
One approach to solving this challenge requires solving the second issue, incorporating active components into a MEMs system. The realization of electrical circuits in single crystal silicon islands supported by thermal isolation bridges is a challenging electrical design and a difficult manufacturing task. Unlike conventional passive MEMs, active MEMs require the formation of the single crystal silicon islands with active components (such as diodes and/or transistors) supported and isolated by thermal insulating bridges.
One approach to creating active MEMs suspends the single crystal absorber element and the intermediate stage islands with thermally isolating bridges. Manufacturing thermally isolating bridges between nano-scale active MEMs components of a bolometer pixel poses unique challenges, in part because active MEMS processing techniques are employed to manufacture the bolometer's pixel components and these processing techniques typically involve some form of etching. Process steps involving a wet etch are particularly troublesome because the surface tension can produce crushing mechanical forces on the delicate nano-scale components in each pixel. The surface tension forces come into play during drying when the etching is completed, and the etching solution is removed. This portion of the process generates strong mechanical forces between the nano-scale components and the substrate. The strong mechanical forces are due to surface tension, and these forces can be sufficiently strong to collapse some of the nano-scale components. The difficulty is compounded with incorporation of active MEMs into the pixel structure, because the manufacturing process must combine active integrated circuit (IC) processing steps and micro-electromechanical systems (MEMs) processing steps, which are often incompatible.